This invention relates generally to navigation techniques and, more particularly, to a method of reducing in-band interference in an Omega navigation receiver employing wide band filters.
It has been customary in the past to employ narrow bandwidth channel filters (approximately 100 Hz bandwidth) to provide adjacent channel rejection of the Omega received frequencies (10.2 KHz, 111/3 KHz and 13.6 KHz). However, these filters, when driven by high level atmospheric noise pulses, will produce a pulse of approximately 14 to 20 milliseconds in width. Since there are many noise pulses spaced from 6 to 10 milliseconds apart, the noise pulses produced in the narrow band filters will overlap, resulting in no time during which clean Omega signals will be available.
It is known that improved performance can be obtained when wide band (1 KHz) channel filters are used in an environment of atmospheric noise. In this case, noise pulses produced in the filter do not overlap, and there is an appreciable amount of time available for receiving clean Omega signals between atmospheric noise pulses, which signals may then be measured and processed.
Experiments have been conducted comparing the performance of an Omega receiving system using, first, a 100 Hz bandwidth channel filter and, second, a 1000 Hz bandwidth channel filter. It was concluded, as a result of these experiments, that the use of a 1 KHz wide channel filter reduces the RMS deviation of a recorded phase measurement by an amount equivalent to improving the received carrier to noise ratio by approximately 10 dB.
The use of wide band channel filters, while reducing atmospheric noise interference presents the additional problem of strong interferring in-band signals. Since there are three basic Omega frequencies at 10.2, 111/3 and 13.6 KHz, plus the possibility of additional frequencies between 11.55 and 13.15 KHz, plus transmissions in this band from other countries, there may exist an interferring signal which, depending on geographical location, can be as much as 60 dB stronger than the desired Omega signal. While it is true that the interferring signal will differ in frequency from the desired Omega signal, this type of interference must be overcome if a successful wide band operation is to be implemented.